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What is a superconductor?

A superconductor is a material that offers no electrical resistance and expels magnetic fields. All known superconductor materials are solids, and only exhibit superconducting properties when cooled to very low temperatures.

With the Type II superconductors used in these experiments, the magnetic field is ejected because of the Meissner effect. Depending on how deep you want to go into the physics of this, read more about the Meissner effect and Quantum Mechanics to learn more.

Why do we need to use liquid nitrogen?

When superconductivity was discovered back in 1911, known superconductors had to be cooled to almost absolute zero (0 K or -273C). The first known superconductors had to be cooled below 10K (-263C) to exhibit superconducting properties.

NOTE: When talking about temperatures this low, the Kelvin scale is often used. The temperature in degrees Kelvin is simply the Celcius temperature plus 273.15. For example, a room temperature of 20C is 293.15K.

In 1986, the first "high temperature superconductor" was discovered, for which the Nobel Price in Physics was awarded in 1987. Note that "high temperature" is a relative term here. Yttrium-barium-copper-oxide, or YBCO, still must be cooled to below 93K (-180C) to act like a superconductor.

Conveniently, YBCO was the first superconductor that can be cooled with liquid nitrogen, which boils at 77K (-196C). This allows for (relatively) easier experimentation and use than previous superconductor materials.

That's why all the superconductors shown here are covered in liquid nitrogen -- to keep their temperature down at 77K (-196C), below the critical temperature at which they start acting like a superconductor.

Magnets can levitate over superconductors! Or superconductors can levitate over magnets!

The video below shows a ring magnet levitating over a series of superconductors arranged in a disc.

This next video below shows a superconductor "train" floating over a bed of neodymium magnets. The train consists of a styrofoam box that contains two superconductor discs, with liquid nitrogen covering them. The track is made of nine BZ084 magnets.

(Special thanks to the folks from Air Products who helped us with the liquid nitrogen, nitrogen safety, and overall liquid nitrogen know-how.)

Why do they levitate? How does it work?

Theoretical magnetic field diagram for a superconducting disc floating over three BZ084 magnets

Superconductors repel magnetic fields due to the Meissner effect. Near the surface of the superconductor material, small currents flow (without any resistance) that make an opposite magnetic field that repels the field from the magnet.

We found that it doesn't behave like a pair of magnets repelling one another. With two magnets repelling, the force varies with how far apart the two magnets are. The closer the magnets, the greater the force.

With the superconductor, we could get levitation to happen 1/8" away or 1/4" away equally well. We also had to hold it there for a moment to get it stable. Why does it levitate in a stable way? Why doesn't the repulsion simply keep pushing away like a pair of magnets would? We found that flux-pinning is a good explanation.

What is flux-pinning?

In the magnetic field diagram shown, the magnetic lines of flux flow from the north pole to the south pole, and do not penetrate the superconductor at all.

At a tiny, microscopic level, there are imperfections in the superconductor. These allow a tiny amount of flux to get through the superconductor, and flow out the other side. These small flows of flux through it is enough to stabilize the superconductor, holding it in place.

Warnings -- don't try this at home!

Here at K&J Magnetics, we're used to warning folks about how strong neodymium magnets can be. Liquid nitrogen comes with a new set of warnings to be mindful of. Lacking experience, we experimented with liquid nitrogen with the help of some folks from Air Products, who are quite experienced with handling this incredibly cold product.

If you experiment with liquid nitrogen, please consult an expert for compete safety instructions. Seek help. Don't try this alone. Some pointers we observed:

This is not a good grammar-school science experiment. Liquid nitrogen is not a toy!

Liquid nitrogen can cause terrible burns. Death of living tissue happens quickly at these temperatures. Wear safety goggles, proper gloves, pants or apron that covers the shoes, etc. Removing jewelry and making sure none gets on top of or down in your shoes is a big concern.

Liquid nitrogen must be stored in a way that allows it to vent. If sealed, it will explode. There are all sorts of specialized containers for handling it.

Use in a well ventilated space. The evaporating nitrogen displaces oxygen, so you could axphyxiate from lack of oxygen. Since nitrogen is odorless, there is no warning.